1. Field of the Invention
The present invention relates to a thermal interface material and a method for making the same, especially to a thermal interface material made from carbon nanotubes and a method for making such a material.
2. Description of Related Art
Nowadays semiconductor devices are smaller and run faster than ever before. These devices also generate more heat than ever before. A semiconductor device should be kept within its operational temperature limits to ensure good performance and reliability. Typically, a heat sink is attached to a surface of the semiconductor device. Heat is transferred from the semiconductor device to ambient air via the heat sink. When attaching the heat sink to the semiconductor device, respective surfaces thereof are brought together into intimate contact. No matter how precisely the heat sink and semiconductor device are manufactured, as much as 99% of the respective surfaces are separated by a layer of interstitial air. Therefore a thermal interface material is used to eliminate air gaps from a thermal interface and to improve heat flow through the thermal interface.
Conventional thermal interface materials are thermally conductive compounds prepared by dispersing a number of thermally conductive fillers into a polymer matrix. The thermally conductive fillers can be graphite, boron nitride, silicon oxide, alumina, and so on. However, polymers have poor thermonductivity. A typical thermal conductivity of a conventional thermal interface material is only about 1 W/mK.
A paper by Nancy Mathis entitled “Thermal conductivity of thin film carbon fiber die attach” (published in RETEC'98 proceedings) discloses a thermal interface material. The thermal interface material binds z-axis carbon fibers in an epoxy matrix. The material can be as little as 76 μm (micrometers) thick, with a thermal conductivity in the range from 140 W/mK to 400 W/mK. However, even this thickness has inherent limitations. Thermal resistance of a thermal interface material is proportional to its thickness. A thickness of less than 40 μm would yeild even higher thermal conductivity.
Savas Berber et al. in “Unusually High Thermal Conductivity of Carbon Nanotubes” (Physics Review Letter, 84, 4613, February, 2000) discloses that a thermoconductivity of an isolated carbon nanotube is 6600 W/mK at room temperature.
U.S. Pat. No. 6,407,922 discloses a thermally conductive material including carbon nanotubes. The thermally conductive material comprises a matrix material and carbon nanotubes incorporated therein. The thermally conductive material includes a first surface adjacent a heat source and a second opposite surface adjacent a heat dissipator. The first surface has a smaller area than the second surface. The carbon nanotubes each extend generally between the first and second surfaces, and splay from the first surface to the second surface. However, the carbon nanotubes are not regularly arranged in the matrix material, and can have poor orientation relative to the first and second surfaces. Consequently, heat cannot be uniformly and efficiently transferred from the first surface to the second surface.